Outside-in Signaling through Integrins and Cadherins: A...

Outside-in Signaling through Integrins and Cadherins:A Central Mechanism to Control Epidermal Growthand Differentiation?Eliane J. Muller1, Lina Williamson1, Carine Kolly1 and Maja M. Suter1

The process of epidermal renewal persists throughout the entire life of an organism. It begins when akeratinocyte progenitor leaves the stem cell compartment, undergoes a limited number of mitotic divisions,exits the cell cycle, and commits to terminal differentiation. At the end of this phase, the postmitotickeratinocytes detach from the basement membrane to build up the overlaying stratified epithelium. Althoughhighly coordinated, this sequence of events is endowed with a remarkable versatility, which enables thequiescent keratinocyte to reintegrate into the cell cycle and become migratory when necessary, for exampleafter wounding. It is this versatility that represents the Achilles heel of epithelial cells allowing for thedevelopment of severe pathologies. Over the past decade, compelling evidence has been provided thatepithelial cancer cells achieve uncontrolled proliferation following hijacking of a ‘‘survival program’’ withPI3K/Akt and a ‘‘proliferation program’’ with growth factor receptor signaling at its core. Recent insights intoadhesion receptor signaling now propose that integrins, but also cadherins, can centrally control theseprograms. It is suggested that the two types of adhesion receptors act as sensors to transmit extracellular stimuliin an outside-in mode, to inversely modulate epidermal growth factor receptor signaling and ensure cellsurvival. Hence, cell–matrix and cell–cell adhesion receptors likely play a more powerful and wide-ranging rolethan initially anticipated. This Perspective article discusses the relevance of this emerging field for epidermalgrowth and differentiation, which can be of importance for severe pathologies such as tumorigenesis andinvasive metastasis, as well as psoriasis and Pemphigus vulgaris.

Journal of Investigative Dermatology (2008) 128, 501–516; doi:10.1038/sj.jid.5701248

Introduction

Epidermal renewal is accompanied bycontinuous reorganization of the kera-tinocyte’s adhesive interactions. This is

particularly evident in the basal layer ofthe epidermis, where the keratinocytesproliferate and, after exiting the cellcycle and commitment to terminal

differentiation, disassemble their cell–-matrix attachments to move upwardsand rebuild the overlying epidermis (forreview see Blanpain and Fuchs, 2006;

& 2008 The Society for Investigative Dermatology www.jidonline.org 501

PERSPECTIVE

Editor’s Note

In this issue of the JID, the Perspectives Series onkeratinocyte adhesion and cell junctions concludes with athought-provoking review of the potential role of integrinsand cadherins as molecules that not only hold thekeratinocytes together, but also serve as signaling proteinsfor the epidermis. In this review, Eliane Muller and colleaguesdiscuss the hypothesis that adhesion proteins may function assignaling molecules that help communicate important extra-cellular signals to the keratinocytes, and by so doing play animportant role in keratinocyte growth and differentiation. This

article will provide readers with a different perspective on thefunction of keratinocyte cell-surface adhesion proteins andstimulate new thought and investigation into how theseproteins may affect epithelial biology.

‘‘To raise new questions, new possibilities, to regard oldproblems from a new angle, requires creative imaginationand marks real advance in science’’. Albert Einstein.

Russell P. Hall, IIIDeputy Editor

Received 6 February 2007; revised 29 September 2007; accepted 10 October 2007

1Molecular Dermatology, Institute of Animal Pathology and DermFocus, Vetsuisse Faculty, University of Bern, Bern, Switzerland

Correspondence: Professor Eliane J. Muller, Molecular Dermatology, Institute of Animal Pathology, Vetsuisse Faculty, University of Bern, Langgass-Strasse 122,Postfach, Bern CH-3001, Switzerland.E-mail: [email protected]

Abbreviations: Dsg, desmoglein; EGF-R, epidermal growth factor receptor; ERK, extracellular signal-regulated kinase; GSK, glycogen synthase kinase; JNK, c-JunN-terminal kinase; MAPK, mitogen-activated protein kinase; NF-kB, nuclear factor-kB; PI3K, phosphatidylinositol trisphosphate kinase; RTK, receptor tyrosinekinase; STAT, signal transducer and activator of transcription

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Watt et al., 2006). These dynamicadhesive interactions could endowepidermal keratinocytes with a uniqueand potent system to survey the chan-ging environment and direct the cellfate when coupled to cell signaling.Hence, this article tackles the emergingknowledge on outside-in signals ema-nating from adhesive interactions inepidermal keratinocytes of the ‘‘transi-tional phase’’, that is from the timepoint the epidermal keratinocyte exitsthe stem cell compartment and be-comes a transit amplifying cell until it isgrowth arrested and has committed toterminal differentiation (Figure 1).

The integrins are the main family ofadhesion molecules that connect kera-tinocytes to the underlying basementmembrane. In human and mouse epi-dermis, the primary integrins expressedare a6b4 (adhering to laminin-332(formerly laminin 5)), a2b1 (adheringto collagen I), a3b1 (adhering tolaminin-332), and the less abundantavb5 (adhering to vitronectin) (Watt,

2002; Wilhelmsen et al., 2006). Ofthese, a6b4 assembles with plectin, BP230, and the collagen-type moleculeBP180 to form hemidesmosomes thattether to keratin intermediate filaments,whereas the other ab-isomers assembleinto focal adhesions that link to actin.Like their suprabasal neighbors, basalkeratinocytes also adhere tightly toeach other via anchoring junctions,that are adherens junctions and desmo-somes (for review see Green andGaudry, 2000; Vasioukhin and Fuchs,2001). The transmembrane proteinsmediating the intercellular adhesiveinteractions belong to the cadherinsuperfamily. In basal keratinocytes,E- and P-cadherin are the majoradhesion-mediating molecules inadherens junctions. They assemblewith plaque proteins p120cnt, b-catenin,plakoglobin (g-catenin), and a-catenininto functional junctions. Alternatively,the strongest intercellular adhesionbetween these cells is provided bydesmosomal cadherins (for example, in

basal keratinocytes desmoglein (Dsg) 3and desmocollin 3 and low levels ofDsg2) forming desmosomes togetherwith plaque proteins such as plakophi-lins and plakoglobin, whereas desmo-plakin anchors desmosomes to theintermediate filament network.

The adhesive interactions of kerati-nocytes are of crucial importance forthe mechanical integrity of the epider-mis, as exemplified by bullous auto-immune diseases that disable adhesivefunctions (for review see Suter et al.,1998; Schmidt and Zillikens, 2000).However, evidence is emerging thatthe functional units of cell adhesion,termed here adhesion receptors, can inaddition serve as transducers of out-side-in signals to control proliferationand terminal differentiation in kerati-nocytes of the transitional phase (forexample, integrins, Mainiero et al.,1997; Nikolopoulos et al., 2005; orcadherins, Pece and Gutkind, 2000;Calautti et al., 2005; Williamson et al.,2006; Perrais et al., 2007; Xie andBikle, 2007).

To date, no enzymatic activity hasbeen attributed to the cytoplasmic tailsof adhesion receptors. Their signalingcapability emanates from the recruit-ment of intracellular signaling compo-nents such as kinases and phosphatasesthat physically link to the adhesionreceptors’ cytoplasmic tail, to adaptorproteins, or plaque proteins (for exam-ple, Fuchs et al., 1996; Mainiero et al.,1996; Shaw et al., 1997; Calautti et al.,2005). Although outside-in signalstransmitted by ligand-activated integrinreceptors have been extensively de-scribed and are known to control cellmigration, survival, and proliferation inmany cell types (for review see Mirantiand Brugge, 2002; Guo and Giancotti,2004; Janes and Watt, 2006; Wilhelm-sen et al., 2006), evidence for outside-in signals triggered by the engagementof cadherin receptors through transad-hesion is only starting to emerge.

Recent studies in various epithelialand endothelial cell types, includingkeratinocytes, now underscore thatcadherins can play a key role incontrolling, for example, cell growth,differentiation, and survival (Takahashiand Suzuki, 1996; Carmeliet et al.,1999; Pece et al., 1999; Cabodi et al.,

Proliferation

Confluency

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Cadherin/EGF-R

EGF-R(RTK)

G1 S

G0

M G2Pro-proliferative

signals

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signals

Integrin/EGF-R

Seeding 3 7–8Days in culture

Ki67/Hoechst Hoechst

Figure 1. Diagram depicting the time frame in which the ‘‘transitional phase’’ takes place in cultured

keratinocytes. Top panel: Left; human epidermis was stained with Ki67 and Hoechst 33258 to show one

proliferating cell per approximately 10 basal keratinocytes; middle; cultured mouse keratinocytes at

confluency were stained with Hoechst 33258 to show asynchronous division; about one cell in 10 is

dividing, similar to what is observed in basal epidermal keratinocytes stained with Ki67. Right: dividing

mouse keratinocytes of the transitional phase are loosely attached to the culture dish. Bottom panel:

Model discussed in this review. After seeding, keratinocytes proliferate under influence of RTK

(specifically discussed for EGF-R) and integrins. Three to four days post-seeding, the keratinocytes reach

confluency. Integrin- and cadherin-mediated signals in cooperation then oppose each other to lead the

keratinocyte into cell-cycle exit, growth arrest, and onset of terminal differentiation. Figure adapted from

Kolly et al. (2005).

502 Journal of Investigative Dermatology (2008), Volume 128

EJ Muller et al.Outside-in Signaling Through Integrins and Cadherins

2000; Pece and Gutkind, 2000;Calautti et al., 2002, 2005; Lapriseet al., 2002, 2004; Dejana, 2004;Qian et al., 2004; Pang et al., 2005;Liebner et al., 2006; Williamson et al.,2006; Hofmann et al., 2007; Kanget al., 2007; Perrais et al., 2007; Xieand Bikle, 2007). Moreover, there isincreasing evidence that cadherins likeintegrins associate with and control theaction of receptor tyrosine kinases(RTKs), in particular of the epidermalgrowth factor receptor (EGF-R) family(integrins, Miranti and Brugge, 2002;Guo and Giancotti, 2004; Janes andWatt, 2006; Wilhelmsen et al., 2006;cadherins, Hoschuetzky et al., 1994;Takahashi and Suzuki, 1996; Pece andGutkind, 2000; Qian et al., 2004;Calautti et al., 2005; Kang et al.,2007; Perrais et al., 2007). Whilesignals generated by ligand-activatedintegrins and RTK are in principle pro-proliferative, signals emanating fromcadherins and RTKs can be pro-prolif-erative or antiproliferative dependingon the cadherin and the associatedRTK; for instance, N-cadherin wassuggested to interact with fibroblastgrowth factor receptor to promoteinvasion and tumor-cell behavior,whereas E-cadherin has been describedto associate/cooperate with EGF-R tomodulate its activity in confluency-dependent events preceding growtharrest (Takahashi and Suzuki, 1996;Pece and Gutkind, 2000; Qian et al.,2004; Calautti et al., 2005; Perraiset al., 2007; Theisen et al., 2007).

Conventional submerged cultures ofmouse, human, and canine keratino-cytes share many features with epider-mal keratinocytes in vivo, and thuswere instrumental in gaining invalu-able insight into the control ofepidermal homeostatic processes (forexample, Yuspa et al., 1989; Pillaiet al., 1990; Poumay and Pittelkow,1995; Kolly et al., 2005). In low-densitycultures and in media containing EGFor related growth factors, keratinocytesproliferate and migrate, and are mostsimilar to keratinocytes in re-epithelia-lizing skin. At confluency, a continuousmonolayer is formed and ample cell–-cell contacts promote the stabilizationof junctional components at the plasmamembrane, similar to the situation in

basal or transit-amplifying keratino-cytes in vivo. At that stage the kerati-nocytes are said to become ‘‘contactinhibited’’, and like transit-amplifyingcells in the epidermis, undergo two tofour mitotic divisions while committingto terminal differentiation (Poumay andPittelkow, 1995; Kolly et al., 2005;Watt et al., 2006). During their mitoticdivisions, the proliferation rate of con-fluent keratinocyte cultures remainsconstant in spite of the steady increaseof antiproliferative factors such ascyclin-dependent kinase inhibitorsp21WAF and p27KIP (Missero et al.,1996; Kolly et al., 2005). This suggeststhat during the transitional phase pro-proliferative and antiproliferativeevents occur simultaneously withinthe same cell, but the antiproliferativeeffectors accumulate over time andonly override pro-proliferative signalswhen they have reached a certainthreshold level (Figure 1; Kolly et al.,2005).

The concept explored in this reviewis that integrins and cadherins sense thecells immediate environment and co-operate in directing the transitionalphase of epidermal renewal by out-side-in signaling. As suggested for otherepithelial cell types, ligand-activatedintegrins and cadherins may therebyserve as signaling receptors, whichexert their function before assemblinginto fully organized junctions firmlyanchoring to the cytoskeletal network(for example, Mariotti et al., 2001;Kovacs et al., 2002; Laprise et al.,2002; Calautti et al., 2005). This con-ceivably facilitates lateral movement,supporting their association and coop-eration with EGF-R family members inaddition to their control.

To begin, this review focuses on thepathways through which integrins canstimulate proliferation, and howgroundbreaking work on b4-integrin-mediated signaling using a novel ap-proach (Nikolopoulos et al., 2005; Guoet al., 2006) integrates with the broaderbody of related literature. The reviewthen continues with an analysis of theemerging knowledge on the involve-ment of cadherins in growth arrest andcommitment to terminal differentiation,and how recent studies on E-cadherin-mediated signaling in keratinocytes

(Calautti et al., 2005; Perrais et al.,2007; Xie and Bikle, 2007) combinewith established results on E-cadherinand new data on Dsg3-mediated sig-naling (Williamson et al., 2006) to forma comprehensive picture, which nowallows us to simultaneously discuss apossible cross talk between integrinsand cadherins with EGF-R family mem-bers in epidermal keratinocytes of thetransitional phase (Figure 1).

Integrin-mediated pro-proliferative signals

Keratinocytes in the basal proliferativelayer of the epidermis can be distin-guished from their suprabasal postmi-totic neighbors by their cell–matrixattachment (Watt, 2002). If this con-nection is disrupted by placing humanor mouse keratinocytes in suspension,they start to terminally differentiate(Green, 1977; Adams and Watt, 1989;Rodeck et al., 1997). Blocking phos-phatidylinositol trisphosphate kinase(PI3K) or alternatively, EGF-R of sus-pension cells, induces apoptosis. In-versely, if integrins are cross-linked,proliferation is maintained (Adams andWatt, 1989; Watt et al., 1993; Rodecket al., 1997; Nikolopoulos et al., 2005).This indicates that normal homeostatickeratinocytes require integrin-mediatedcell-substrate adhesion to proliferate.Moreover, when cell–substrate adhe-sion is lost, differentiation-promotingand cell survival cascades are acti-vated, which depend on PI3K and EGF-R. Collectively, these findings suggestthat in epidermal keratinocytes, func-tional integrin receptors either supportproliferation or alternatively suppressterminal differentiation or both.

Reports on the lack of a proliferativephenotype in b1-integrin-knockout ker-atinocytes in vitro and more recently inconditional Cre-mediated b1-integrin-knockout epidermis of adult miceappeared to exclude b1-integrin as aregulator of the proliferative processduring epidermal turn-over (Raghavanet al., 2003; Lopez-Rovira et al., 2005).Moreover, deletion confined to smallstretches (to prevent loss of cell–matrixadhesion) of b4-integrin, the secondmajor b-subunit in the epidermis, alsofailed to impede normal proliferationand/or terminal differentiation, raisingfurther doubts about a pro-proliferative

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activity of integrins during epidermalrenewal (Raymond et al., 2005). Thesefindings were surprising in view of themany signaling events reported down-stream of integrin receptors (for reviewsee Miranti and Brugge, 2002; Guo andGiancotti, 2004; Janes and Watt, 2006;Wilhelmsen et al., 2006).

Studying the role of adhesion recep-tors is complicated by the fact that theirgenetic deletion not only affects signal-ing but also adhesion. In addition,deletion of adhesion components inskin can trigger secondary events suchas inflammation, which induce hyper-proliferation as a bystander effect. Asan example, varying grades of inflam-mation were discussed as a reason forconflicting results obtained from differ-ent b1-integrin-knockout mice (Lopez-Rovira et al., 2005). Consequently, formany studies on adhesion receptors, itis difficult to discriminate betweensignals related to homeostatic pro-cesses, and those linked to lack oftissue integrity or to secondary eventslike inflammation. In the case of b4-integrin, this problem was recentlyelegantly circumvented by expressinga C-terminal deletion mutant (from aa1,355 onwards) in mouse epidermis,which is not critical for adhesion butabrogates signaling (Nikolopouloset al., 2005).

Transgenic mice (both newbornand adult) expressing the signaling-defective b4-integrin mutant showed atwofold decrease in the epidermalproliferative index, confirming thatb4-integrin actually contributes to pro-liferation (Nikolopoulos et al., 2005).Furthermore, keratinocytes from b4-integrin mutant mice failed to prolifer-ate and migrate in vitro in response toEGF, suggesting an essential coopera-tion between b4-integrin and an RTK inproliferative epidermal keratinocytes.The follow-up of this study with com-parative expression of full-length andsignaling-defective b4-integrin in atransgenic mouse mammary carcinomamodel with constitutive ErbB2 activa-tion (an EGF-R family member; Ishiza-war and Parsons, 2004), confirmed thepro-proliferative activity of b4-integrin;that is, loss of b4-integrin signaling-suppressed tumor onset, and invasivegrowth. More importantly, this study

demonstrated that b4-integrin has tophysically associate with an EGF-Rfamily member to amplify pro-prolif-erative signals, and provided novelinsights into the mechanism of the b4-integrin-mediated cooperation (Guoet al., 2006). Specifically, after associa-tion, b4-integrin mediates the phos-phorylation of ErbB2 on Tyr877 by aSrc-family kinase. This phosphorylationsite resides in the catalytic domain ofErbB2 and corresponds with Tyr845 inEGF-R, which is known to be essentialfor mitogenesis in terms of DNA synth-esis (Ishizawar and Parsons, 2004).

The combination of the more de-tailed study in mammary epitheliumwith the findings in mouse epidermisusing the unique approach of signaling-defective (but adhesion competent) b4-integrin now enables a clear definitionof signals relevant for the b4-integrin-mediated pro-proliferative and pro-sur-vival activity (Nikolopoulos et al.,2005; Guo et al., 2006). Like inmammary epithelium, b4-integrin wasfound to cooperate with EGF-R inepidermal keratinocytes to enhanceproliferation in a Src- but not Ras-dependent manner. Signaling-wise thisinvolves mainly the mitogen-activatedprotein kinase (MAPK) kinase moduleJNK/c-Jun and contributing activationof signal transducer and activator oftranscription 3 (STAT3), whereas thePI3K/Akt kinase survival axis (proteinkinase B) was suggested to be activatedby b4-integrin and/or RTK alone (seeFigure 2, figure legend for full namesof effectors). The relevance of thesesignals for the processes governingepidermal proliferation and cell survi-val is, discussed in more detail in thefollowing three sections.

MAPK activation downstream ofb4-integrin/EGF-R: pro-proliferativeevents. In epithelial cells, three MAPKmodules have been identified, whichlead up to extracellular signal-regu-lated kinase (ERK)1/2, c-Jun N-terminalkinase (JNK), or p38 activation, respec-tively (Cowan and Storey, 2003). Ofthese, JNK-activated c-Jun was found tocentrally control the proliferative activitydownstream of b4-integrin and EGF-Rin mouse and human keratinocytesplated on laminin-332, as well as

downstream of b4-integrin/ErbB2 inthe mammary carcinoma model (Main-iero et al., 1997; Nikolopoulos et al.,2005; Guo et al., 2006; Figure 2).Importantly, both b4-integrin/EGF-Rand b4-integrin/ErbB2 did not acti-vate/phosphorylate JNK, but insteadtriggered the nuclear translocation ofactivated JNK. Although the mecha-nism for nuclear targeting is currentlynot known, it implies b4-integrin-in-dependent JNK activation through EGF-Ror another RTK (Zenz and Wagner,2006), followed by b4-integrin/EGF-R-dependent nuclear targeting (Figure 2).

An important role of JNK in epider-mal proliferation is compatible with thecontribution of JNK1 (but not of JNK2;Sabapathy and Wagner, 2004; Westonet al., 2004) and c-Jun to normalepidermal proliferation as well aspathological conditions like psoriasisand tumor formation (Zenz andWagner, 2006). Consistently, JNKdrives proliferation in embryonicmouse epidermis with NF-kB/RelAdeficiency (Zhang et al., 2004),whereas experimentally blocked NF-kBwas found to act in a laminin-332and b4-integrin-dependent manner topromote human epidermal tumorigene-sis in cooperation with oncogenic Ras(Dajee et al., 2003). In contrast to theinvolvement of NF-kB blockade intumorigenesis, activation but not inhi-bition of NF-kB/RelA was implicated inb4-integrin/EGF-R-dependent prolifera-tion in addition to JNK/c-Jun in mousekeratinocyte cultures, raising the possi-bility that NF-kB can function in abidirectional manner in epidermal ker-atinocytes (Nikolopoulos et al., 2005).

In parallel to JNK, two other MAPKs,ERK1 and ERK2, were also found to betargeted to the nucleus in a b4-integrin-dependent manner in keratinocytes(Nikolopoulos et al., 2005). Nonethe-less, proliferation was not dependenton ERK in either keratinocytes or themammary carcinoma model down-stream of b4-integrin (Nikolopouloset al., 2005; Guo et al., 2006). This isin line with persistence of hyperproli-feration in NF-kB-blocked human ker-atinocytes treated with pharmaco-logical ERK inhibitors (Zhang et al.,2004), as well as lack of evidence for acontribution of ERK to mitogenesis



downstream of Tyr845EGF-R (Ishizawarand Parsons, 2004).

Janus kinase–STAT activation down-stream of b4-integrin/EGF-R: prolifera-tion-dependent events. In confluentmammary carcinoma cells in vitro, b4-integrin/ErbB2 activation prevented con-tact-induced growth inhibition (Guoet al., 2006). Simultaneously, adherensjunctions and tight junctions were dis-rupted in spite of the fact that expressionlevels of junctional proteins remainedunaffected. Junctional disruption wasforestalled by signaling-defective b4-in-tegrin or by inhibitors to ErbB2, and inpart by a dominant-negative STAT3mutant. This indicates that in additionto supporting proliferation via JNK, b4-integrin/ErbB2 cross talks to intercellularjunctions via STAT3 in the mammarycarcinoma cells (Figure 2).

b4-integrin STAT3-induced junc-tional disruption has as yet not beenaddressed in other cell types. It ishowever of particular interest as itdiffers from endocytosis and down-regulation of junctional componentsknown to accompany Src/PI3K-in-duced snail-related tumorigenic trans-formations (Thiery, 2002). It insteadresembles normal mitosis-induced dis-ruption and endocytosis of adherensjunctions, which occurs without degra-dation of junctional proteins in dividingMadin–Darby canine kidney cells atconfluency (Bauer et al., 1998). Con-sistent with the activation of b4-integrinsignaling during mitotic processes(Nikolopoulos et al., 2005; Guo et al.,2006), cell–substrate adhesion was re-duced in dividing Madin–Darby caninekidney cells. Reduced cell–substrateadhesion is a long known phenomenon

of dividing cells and is also observed inconfluent mouse keratinocyte cultures(Figure 1, top). In several epithelial celltypes, including HaCaT keratinocytes,reduced cell-substrate adhesion wasascribed to Src-family kinase-induceddisassembly of hemidesmosomes pre-ceding b4-integrin signaling (Mariottiet al., 2001), a phenomenon that mightalso account for Madin–Darby caninekidney cells. Disassembly of hemides-mosomes was discussed to uncouplea6b4-integrin from the adhesion com-plex and from the underlying cytoske-letal, to favor its colocalization withEGF-R in signaling platforms known aslipid rafts (Mariotti et al., 2001; Gag-noux-Palacios et al., 2003).

In vivo, b4-integrin amplifies pro-proliferative signals in basal epidermalkeratinocytes (Nikolopoulos et al.,2005), where STAT3 is expressed and

Intercellular space

Cytoplasm E-cad

STAT3 STAT3

STAT3 JNK1

JNK1 JNK2

JAK

PI3K

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EGF-R EGF-R E-cad

Basement membrane

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JNK1 c-Myccyclins

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p120RhoA

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PG

PG

β-cat

β-cat

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α6 β4

GSK3β

Figure 2. Opposing signaling events discussed in this review to emanate from b4-integrin and E-cadherin receptors within one keratinocyte. Depicted are

effectors contributing to proliferation (b4-integrin) and cell-cycle exit (E-cadherin), in addition to cell survival in keratinocytes. Note that the respective cascades

were investigated independently of each other but under similar experimental settings. JNK/c-Jun was found to be crucial for proliferation. STAT3 was described

to contribute to the disruption of tight junctions and adherens junctions in mammalian carcinoma cells (Guo et al., 2006). Akt generates survival signals, in

addition to possibly contributing to proliferation and terminal differentiation depending on the receptor and Akt isoform. Yellow star on JNK1: b4-integrin/EGF-R-

mediated nuclear targeting of phosphorylated JNK1; purple stars on EGF-R: Src kinase-mediated phosphorylation of EGF-R. Abbreviations: b-cat, b-catenin;

EGF-R, epidermal growth factor receptor; ERK1/2, extracellular signal-related kinase; GSK3, glycogen synthase kinase; c-Jun, Janus kinase; JNK, c-Jun

N-terminal protein kinase; PDK-1, phosphoinositide-dependent kinase-1; PG, plakoglobin; PI3K, phosphatidylinositol triphosphate kinase; Srcasm,

Src-activating and signaling molecule; STAT; signal transducers and activator of transcription.




can be activated by EGF/EGF-R (Nishioet al., 2001; Chan et al., 2004; Li et al.,2007). This raises the possibility of ab4-integrin/EGF-R to cadherin crosstalk via STAT3 also in vivo. Althoughthis needs to be confirmed, transientdissolution of intercellular junctionswithout degradation of junctional pro-teins would perfectly meet the require-ments of dividing keratinocytes inepidermal tissue. These cells need torapidly reorganize their cell–matrix andcell–cell attachment to preserve theintegrity of the epidermal sheet. Suchreorganizational processes would how-ever only concern one dividing cell perepidermal proliferative unit (10–20basal keratinocytes; Figure 1; Gambar-della and Barrandon, 2003), and pos-sibly only in selected areas of theplasma membrane. Consistently, in-spection of basal keratinocytes byelectron microscopy did not revealapparent junctional disruption (Bakerand Garrod, 1993). Altogether, thissuggests that the b4-integrin/EGF-R/STAT3-induced junctional disruptionwithout degradation could be a generalmechanism, which reversibly disas-sembles intercellular junctions duringmitosis (Figure 2). This would furtherpermit b4-integrin to simultaneouslycontrol pro-proliferative signals andSTAT3-related antiproliferative signalsthat emanate from transadheringE-cadherin (as will be discussedbelow).

PI3K/Akt activation downstream ofb4-integrin/EGF-R: survival and poten-tial pro-proliferative events. Akt is aserine/threonine kinase with a well-known role in cell survival (for reviewsee Vivanco and Sawyers, 2002).Accordingly, Akt is activated and ensurescell survival in keratinocytes devoid ofcell–matrix attachment (Frisch andScreaton, 2001). In addition to its pro-survival function, Akt has also beeninvolved in cellular processes rangingfrom migration, proliferation, to term-inal differentiation (for review seeVivanco and Sawyers, 2002; Tokerand Yoeli-Lerner, 2006). These func-tions might depend on the Akt isoformand the activating receptor. Class IAPI3K, the main activators of Akt, can berecruited to receptors such as activated

RTK (for review see Vivanco andSawyers, 2002; Engelman et al.,2006), integrins (Mainiero et al.,1997; Shaw et al., 1997; Miranti andBrugge, 2002), but also transadheringcadherins (as further discussed below).

As outlined above, b4-integrin sig-naling involves reduced cell–substrateadhesion secondary to disruption ofhemidesmosomes (Mariotti et al.,2001). Accordingly, Akt is activated/phosphorylated to ensure survival inmouse keratinocytes and mammarycarcinoma cells with signaling activeb4-integrin, and this activation in partdepends on RTK (Nikolopoulos et al.,2005; Guo et al., 2006). In case ofsignaling-defective b4-integrin, cellularstress such as starvation or enhancedproliferation resulted in apoptosis inboth cell types. This implies that undercircumstances requiring high Akt activ-ity, such as stress, the Akt poolbecomes limiting in absence of b4-integrin signaling. Hence, this providescircumstantial evidence that b4-integ-rin itself can activate Akt in theseexperimental settings. A possibility,which so far lacks experimental proof,is that, in addition to supporting cellsurvival, b4-integrin-mediated Akt acti-vation might be involved in nucleartargeting of JNK and thereby contributeto the proliferative process. This iscompatible with the observation thatAkt can amplify growth factor-inducedsignals (Toker and Yoeli-Lerner, 2006).

In normal homeostatic mouse epi-dermis, Akt activation is mainly con-fined to suprabasal cells, with theexception of scattered keratinocytes inthe basal layer with a loosely attachedappearance (Calautti et al., 2005).Overall, this is consistent with the factthat Akt is activated in keratinocyteswithout cell–substrate attachment topromote cell survival (Frisch and Screa-ton, 2001). Hence, Akt-positive cells inthe basal layer can comprise keratino-cytes committed to terminal differentia-tion that have disassembled theirhemidesmosomes to start migratingupwards. Alternatively, as outlinedabove, Akt can be activated in activelydividing cells that have disassembledtheir hemidesmosomes in presence ofb4-integrin signaling. In the basal layer,the Akt-positive cells could therefore

also result from and indicate b4-integ-rin signaling (one out of 10–20 cells inthe epidermis; Figure 1), a possibility,which warrants further investigations.

Conclusion on a6b4-integrin signalingin epidermal keratinocytes. In sum-mary, in epidermal keratinocytes ofthe transitional phase (Figure 1),ligand-activated b4-integrin receptorscooperate with EGF-R to amplifypro-proliferative and survival signals.This occurs in an Src- but notRas-dependent manner and mainlyinvolves JNK and Akt (Figure 2). Thecooperation between b4-integrin andEGF-R may be brought about byuncoupling a6b4-integrin from hemi-desmosomes, which facilitates the as-sociation with EGF-R in lipid rafts(Mariotti et al., 2001). In addition topro-proliferative signals, b4-integrin/EGF-R may control antiproliferativeevents emanating from transadheringE-cadherin receptors via STAT3(Figure 2). The collective findings onb4-integrin signaling in keratinocytesare supported by complementary ob-servations in a mammary carcinomamodel, while the evoked signalingpathways align with current knowledgeon pro-proliferative and survival signalsdriving epidermal renewal.

With respect to the mechanisms, anumber of important questions remainto be answered, including how JNK istargeted to the nucleus, whether STAT3is activated in a b4-integrin/EGF-R-dependant manner in the epidermisand whether it contributes to junctionaldisassembly, as well as to which extentAkt is involved in the pro-proliferativeprocess.

Based on the studies using signaling-defective b4-integrin, the central query‘‘does b-4-integrin-mediated signalingamplify epidermal proliferation in vivo’’seems to have been answered in theaffirmative. Outstanding is whether andto which extent other b-integrin subunitsand eventually also a6-integrin (Owenset al., 2003) contribute to this event,and whether they act synergistically orredundantly with b4-integrin. Redun-dancy is actually suggested by thediscrepant observation that the signal-ing-defective b4-integrin but not itsgene deletion displays a proliferative



phenotype in the epidermis (Nikolopou-los et al., 2005; Raymond et al., 2005).Signaling-defective b4-integrin mightcompete with wild-type integrins for thebinding of essential proliferation-promot-ing factors, without being able to propa-gate their activity. This conceivably failsto happen in the knockout situation.Redundancy is also supported by the factthat the amplification of pro-proliferativesignals in the manner described forb4-integrin equally applies for b1-integrin.b1-integrin phosphorylation by Srcresults in EGF-R activation (Miranti andBrugge, 2002), and b1-integrin expressionis required for the initiation of mousemammary tumors expressing the polyomavirus middle T antigen (White et al.,2004). Furthermore, both, b4- andb1-integrin amplify signals of other RTKsthan of ErbB2 and EGF-R, such as RON,the hepatocyte growth factor receptorMet, and tumor necrosis factor receptor,and all have been involved in carcino-genesis also in the skin (for review seeMiranti and Brugge, 2002; Guo andGiancotti, 2004; Janes and Watt, 2006).Collectively, these results strongly arguein favor of an obligatory role of theintegrin family members in controllingand propagating pro-proliferative andsurvival signals in basal keratinocytes ofthe transitional phase.

E-cadherin-mediated antiproliferativeevents

In contrast to integrins, evidence forcadherin-induced outside-in signalingcame into focus only slowly. Publishedover the last 10 years, a number ofstudies in both epithelial and endothe-lial cells now appear to agree thatsignaling cascades emanating fromtransadhering or ligated E-cadherin orvascular (V) E-cadherin play an impor-tant role in confluency-dependentgrowth arrest and differentiation (Taka-hashi and Suzuki, 1996; Carmelietet al., 1999; Pece et al., 1999; Cabodiet al., 2000; Pece and Gutkind, 2000;Calautti et al., 2002, 2005; Lapriseet al., 2002, 2004; Qian et al., 2004;Liebner et al., 2006; Hofmann et al.,2007; Kang et al., 2007; Perrais et al.,2007; Xie and Bikle, 2007). Proof ofconcept that E-cadherin actually acts asan adhesion receptor in keratinocytesas well as in other epithelial cells, was

provided by homophilic ligation ofE-cadherin with recombinant protein,which results in direct activation ofsignaling cascades and growth arrest(Kovacs et al., 2002; Pang et al., 2005;McLachlan et al., 2007; Perrais et al.,2007), and inversely, by function dis-rupting E-cadherin antibodies, whichabrogate for instance E-cadherin signal-ing, growth arrest, or terminal differ-entiation (Takahashi and Suzuki, 1996;Pece et al., 1999; Noren et al., 2001;Laprise et al., 2002; Qian et al.,2004; Calautti et al., 2005; McLachlanet al., 2007).

As far as analyzed, the mode bywhich E-cadherin transmits growth-in-hibiting outside-in signals seems tofollow a strikingly similar scheme thanthat of integrins involved in prolifera-tion. Discussed in more detail below,signaling is initiated through homophi-lic transadhesion between E-cadherinmolecules at points of cell–cell contact(Figure 2). The E-cadherin complexthen associates and cooperates withan EGF-R family member to activatethe PI3K/Akt pathway and possiblyMAPK in a Src-family kinase-depen-dent manner, as indicated in a numberof complementary studies on keratino-cytes and other cell types (Hoschuetzkyet al., 1994; Takahashi and Suzuki,1996; Pece and Gutkind, 2000; Qianet al., 2004; Calautti et al., 2005;Perrais et al., 2007). Ultimately, thisresults in the loss of the cells’ respon-siveness to EGF through inhibition ofthe EGF-R pro-mitotic activity. Re-cently reported in A431 cells, this canoccur through E-cadherin-mediatedinhibition of transphosphorylation onTyr845EGF-R (Perrais et al., 2007),the site, which catalyzes EGF-inducedmitogenesis (Ishizawar and Parsons,2004).

As outlined above also, studies inhuman and mouse keratinocytes high-light the involvement of E-cadherin inoutside-in signaling. In addition, theydemonstrate cooperation between E-cadherin and EGF-R in activating PI3K/Akt and inducing growth arrest (Fig-ure 2; Pece and Gutkind, 2000; Ca-lautti et al., 2005; Perrais et al., 2007;Xie and Bikle, 2007). Although not yetconfirmed more broadly, this perfectlyfits the observations that inhibition of

PI3K or EGF-R is sufficient to preventterminal differentiation in keratinocytesin suspension (Rodeck et al., 1997;Nikolopoulos et al., 2005), and thatfunction disrupting antibodies toE-cadherin abrogate Akt phosphorylationinducing growth arrest and terminaldifferentiation in conventional sub-merged mouse keratinocyte cultures atconfluency (Calautti et al., 2005). Theunderlying mechanism of the apparentjoint signaling through E-cadherin/EGF-Ris currently not known. In mousekeratinocytes, it was however shownto result in Src kinase-mediated phos-phorylation of EGF-R, presumably onTyr920 (Calautti et al., 2005). This cangenerate a docking site for p85, aregulatory subunit of PI3K (Stoveret al., 1995; Calautti et al., 2005), withpotential activation of diverse signalingcascades (for review see Engelmanet al., 2006), including the MAPKdescribed in human HaCaT cells (Peceand Gutkind, 2000).

In support of a possible involvementof E-cadherin in growth control inkeratinocytes in vivo, E-cadherin-knockout epidermis displays somehyperproliferation, which howeverappears to be limited due to compen-satory upregulation of P-cadherin(Young et al., 2003; Tinkle et al.,2004; Tunggal et al., 2005) or (aswill be further discussed below)compensatory desmosomal cadherins.Furthermore, compatible with a dualcontribution of EGF-R, PI3K, and Akt topro-proliferative and antiproliferativeevents in epidermal keratinocytes (Fig-ure 2), EGF-R- and Akt1/2-knockoutmice as well as mouse keratinocytesoverexpressing PI3K class IA catalyticsubunits display a broad range ofepidermal phenotypes, which extendfrom impaired migration and prolifera-tion to impaired terminal differentiation(Miettinen et al., 1995; Peng et al.,2003; Pankow et al., 2006).

PI3K/Akt activation downstream ofcadherins/EGF-R: antiapoptotic andantiproliferative events. In keratino-cytes, exogenous expression ofmembrane tethered, constitutivelyactive Akt was found to be sufficientin driving cell-cycle exit and onset ofterminal differentiation, underscoring




the importance of Akt in this process(Calautti et al., 2005). Accordingly, Aktactivation downstream of E-cadherinengagement has been analyzed in quitesome detail in mouse and humankeratinocytes (Figure 2). The sequenceof events outlined in the following wasmainly established in several follow-upstudies in mouse keratinocytesby Calautti and co-workers, andwas sequentially confirmed by otherinvestigators for mouse or humankeratinocytes.

What is known so far is that trans-adhering E-cadherins are stabilizedthrough association with p120cnt (forreview see Anastasiadis, 2007), whichresults, in the context of confluentmouse keratinocytes, in recruitmentand activation of the small GTPaseRhoA (Calautti et al., 2002; Anastasia-dis, 2007) necessary for the associationof a Src-family tyrosine kinase with thenascent cadherin tails at the plasmamembrane (Cabodi et al., 2000; Ca-lautti et al., 2002). Src-family kinasesthen phosphorylate the plaque proteinsb-catenin, plakoglobin, p120cnt, and a-catenin, which is required for theirassociation with the E-cadherin tail(Calautti et al., 1998, 2005). In mousekeratinocytes, Src kinase-mediatedphosphorylation was found to generatea docking site for PI3K (p85) onplakoglobin in an EGF-R-dependentmanner, to a minor extent on p120cnt,but not on b-catenin, which apparentlylacks a PI3K (p85)-docking motive(Calautti et al., 2005). Consistently, inthe former study the catalytic subunit ofPI3K (p110) was mainly associatedwith plakoglobin upstream of PI3K/Aktactivation. This contrasts with datafrom human keratinocytes in which b-catenin and p120cnt but not plakoglo-bin were found to be involved in PI3Krecruitment to transadhering E-cadher-in upstream of Akt (Xie and Bikle,2007). On the other hand, PI3K recruit-ment was related to b-catenin but notp120cnt in SW480 human colon carci-noma cells (Perrais et al., 2007).

The discrepant results on the invol-vement of b-catenin or plakoglobin intethering PI3K to the E-cadherin tail inkeratinocytes might be explained by afundamental difference in the experi-mental set ups (Calautti et al., 2005;

Xie and Bikle, 2007). In the context ofhuman keratinocytes, PI3K associationwas investigated in the first minutesafter E-cadherin engagement, whereasit was addressed 1 hour later in mousekeratinocytes (Calautti et al., 2005; Xieand Bikle, 2007). Hence, it is concei-vable that the PI3K recruitment occursin a biphasic time course, at first forE-cadherin-mediated signaling involvingp120cnt and b-catenin or b-catenin-dependent molecules, and then secondfor Dsg3-mediated signaling involvingplakoglobin (SID Meeting abstract;Schulze et al., 2007), while potentiallyunderlying the cross talk betweenE-cadherin and desmosomal cadherinsthat plakoglobin was described tomediate many years ago (Lewis et al.,1997). Although this hypothesis needsto be confirmed, it is noteworthy thatquite a few studies on various mousecell types have concluded on theassociation of PI3K with b-cateninbased on co-immunoprecipitation ex-periments using E-cadherin antibodies(for example, Espada et al., 1999;Woodfield et al., 2001). Given thatplakoglobin and b-catenin coexist inadherens junctions (Butz et al., 1992),they co-precipitate and hence cannotbe distinguished (for example, Calauttiet al., 2005). Although the dockingpartners for PI3K on E-cadherin inkeratinocytes require further detailedinvestigations, the studies on humanand mouse keratinocytes agree on Aktactivation following PI3K recruitmentto transadhering E-cadherin (Calauttiet al., 2005; Xie and Bikle, 2007).

Two possible targets inhibitedthrough Akt-induced phosphorylationare glycogen synthase kinase (GSK3) aand b (for review see Vivanco andSawyers, 2002). This inhibition isknown to promote, among many otheractivities, the stabilization and nucleartranslocation of b-catenin and plako-globin in various cell types (Aberleet al., 1997; Kodama et al., 1999).Accordingly, confluent mouse and hu-man keratinocytes subjected to Aktactivation exhibit phosphorylatedGSK3a and GSK3b (Calautti et al.,2005; Thrash et al., 2006), which mighttrigger the confluency-dependent trans-location of b-catenin and plakoglobininto the nucleus that was described in

an unrelated study on mouse keratino-cytes (Figure 2; Williamson et al.,2006). One of the reported conse-quences of this translocation is theplakoglobin-mediated suppression ofthe pro-proliferative proto-oncogenec-Myc, which is critical for growthinhibition in these cells (Arnold andWatt, 2001; Waikel et al., 2001; Kollyet al., 2005; Figure 2). Consistently,maximal accumulation of nuclear pla-koglobin as well as nuclear b-cateninchronologically correlates with re-duced cyclin D1 levels and keratino-cyte growth arrest (Williamson et al.,2006). It is noteworthy that nuclearaccumulation of plakoglobin and/orb-catenin has not yet been functionallylinked to transadhering E-cadherin,plakoglobin was however related toDsg3-induced signaling as will befurther discussed below (Williamsonet al., 2006).

Besides b-catenin and plakoglobin,another potential effector activateddownstream of E-cadherin/Akt andGSKb is NF-kB/RelA, which also cancontrol cell-cycle exit in human andmouse keratinocytes (Dajee et al.,2003; Zhang et al., 2004). Upstreamof Akt, E-cadherin/PI3K activates phos-pholipase-C-g1, which is a crucialelement in the mobilization of intracel-lular calcium and terminal differentia-tion (Xie and Bikle, 2007). The relativecontribution of the Akt pathway andPI3K/phospholipase-C-g1 to growtharrest versus terminal differentiationin keratinocytes downstream oftransadhering E-cadherin, remains tobe investigated.

MAPK activation downstream of cad-herins/EGF-R: potential antiprolifera-tive events. Only a small number ofreports have investigated the activationof MAPK modules downstream oftransadhering E-cadherin and cooperat-ing EGF-R in keratinocytes and otherepithelial cells. However, some evi-dence exists that ERK1/2 and p38 aretransiently activated (Pece and Gut-kind, 2000; Laprise et al., 2004). Inhuman HaCaT keratinocytes for in-stance, ERK1/2 phosphorylation in-creases in dependence of EGF-Rrecruitment to transadhering E-cadher-in, and E-cadherin-mediated activation



of MAPK was also reported in intestinalepithelial cells (Pece and Gutkind,2000; Laprise et al., 2004). Of particu-lar interest is that JNK2 was found to benecessary for cell-cycle arrest in kera-tinocytes, as exemplified by hyperpro-liferation of JNK2-knockout epidermis(Sabapathy and Wagner, 2004; Westonet al., 2004). In light of the central pro-proliferative role attributed to JNK/c-Jun downstream of b4-integrin/EGF-Rin keratinocytes (Nikolopoulos et al.,2005), it is noteworthy that JNK2 wassuggested to destabilize JNK1-inducedc-Jun (Figure 2). As E-cadherin ligationor transadhesion is sufficient for growtharrest and onset of terminal differentia-tion in cultured keratinocytes (Calauttiet al., 2005; Perrais et al., 2007; Xieand Bikle, 2007), JNK2 might poten-tially be activated via Src kinase-mediated phosphorylation of EGF-Ron Tyr920 (Calautti et al., 2005), tocounteract the pro-proliferative c-Junactivity, a hypothesis requiring furtherdetailed investigations.

Inhibition of ERK, Janus kinase/STAT3,and JNK1: antiproliferative events.E-cadherin signaling seems to be ableto oppose keratinocyte proliferation bytransient activation of the PI3K/Akt/GSK3b axis (Calautti et al., 2005;Figure 2). It can also contribute todestabilizing JNK1-induced c-Jun byJNK2 (Sabapathy and Wagner, 2004),and to silencing the mitotic activityof EGF-R by inhibiting Tyr845 phos-phorylation (Perrais et al., 2007).Furthermore, in human keratinocytes,E-cadherin/PI3K also activates theterminal differentiation-inducing phos-pholipase-C-g1 (Xie and Bikle, 2007),whereas the Src tyrosine kinase Fyn canactivate protein kinase-Cd, which wassuggested to be the major proteinkinase-C isoform involved in terminaldifferentiation downstream of EGF-R(Denning et al., 1995). The latterdownregulates, for example, a6b4-in-tegrin expression, an event necessary tosupport the upward migration of kera-tinocytes (Alt et al., 2004). Further-more, the increasing activity of theSrc-family kinase Fyn measured duringthe transitional phase (Calautti et al.,1998, 2002) was found to exert anegative feedback on EGF-R-triggered

pro-proliferative effectors via the Src-activating and signaling molecule(Srcasm) in mouse keratinocytes (Liet al., 2007). Downstream of EGF-Ractivation, Srcasm was shown to blockERK1/2 and STAT3 in addition to PDK-1, the latter being a crucial activator ofthe PI3K/Akt axis (Vivanco and Saw-yers, 2002). In other terms, Srcasmappears to be able to inhibit all pro-proliferative signals described down-stream of integrin/EGF-R, except c-Jun,which was not addressed and might bedestabilized by JNK2 (Figure 2). Finally,Srcasm itself exerts a negative feed-back, as it inhibits its activator, the Srckinase Fyn (Li et al., 2007). Althoughnot investigated so far, this couldpotentially terminate both b4-integrinand E-cadherin-mediated signaling,which in each case is transientand depends on Src-family kinases(Figure 2). Following inhibition ofSrc-family kinases, phosphatases likelydephosphorylate adherens junctioncomponents to allow their assemblyinto stable junctions at the plasmamembrane (Gumbiner, 2000), similarlyas was suggested for the termination ofb4-integrin-mediated signaling in Ha-CaT keratinocytes (Mariotti et al.,2001).

It has long been known that Src-mediated tyrosine phosphorylationshifts cadherin-based cell adhesionfrom a strong to a weak adhesive state(Takeda et al., 1995; for review seeGumbiner, 2000). As Src activation is aprerequisite for E-cadherin-induced sig-naling (Calautti et al., 2005), this wouldimply that signaling active E-cadherinreceptors are of an intermediate statebetween no and strong intercellularadhesion. The transient nature of sig-naling cascades triggered by E-cadherinactually entails the formation of anintermediate state and would bestexplain the bidirectional activity ofSrc-family kinases that are involved injunction assembly as well as disassem-bly (Takeda et al., 1995; Calautti et al.,1998; McLachlan et al., 2007).

Indeed, based on observations inChinese hamster ovary and intestinalcells by immunofluorescence analyses,signaling activity was assigned toE-cadherin molecules without cytoskeletalanchorage because E-cadherin-mediated

signaling preceded the assembly ofadherens junctions (Kovacs et al.,2002; Laprise et al., 2002). A similarpossibility was discussed for mousekeratinocytes (Calautti et al., 2005)and would correspond with the proper-ties evoked for the signaling-active b4-integrin pool (Mariotti et al., 2001).Although this topic needs to be ad-dressed in detail, the possibility that thesignaling pool of E-cadherin residesoutside of junctions and has no orweak cytoskeletal connection is com-patible with the notion that transadhe-sion between E-cadherins (anddesmosomal cadherins) does not ne-cessarily involve cytoskeletal ancho-rage (Roh and Stanley, 1995; Marcozziet al., 1998). In addition, thecapacity of sensing the environmentby ‘‘ultrafast probing’’ was attributed toC-cadherin, which has no cytoskeletalinteraction site (Pierres et al., 2007),while the lateral movement thought tofacilitate the association of E-cadherinwith EGF-R in lipid rafts (Seveau et al.,2004), was ascribed to E-cadherinreceptors without cytoskeletal ancho-rage, which reside outside of cell–cellcontacts in a mouse keratinocyte cellline (Kusumi et al., 1993). Support forthe possibility that non-junctionalE-cadherin without strong cytoskeletalanchorage exists in vivo was providedby the observation that E-cadherinantibodies bind to the plasma mem-brane between adherens junctions inmouse epidermis (Boller et al., 1985).

Conclusion on E-cadherin/EGF-R sig-naling in epidermal keratinocytes.Compelling but so far limited studiesimplicate transadhering E-cadherinreceptors and cooperating EGF-R inoutside-in signaling during growtharrest and terminal differentiation inkeratinocytes, principally via PI3K/Aktand potentially MAPK (Figures 1 and 2).More ample results from otherepithelial cell types and also fromV(E)-cadherin in endothelial cells cor-roborate these findings, suggesting apotent cadherin-mediated mechanismto sense cell–cell contact and regulatecell fate. The capability to sense theenvironment might be an attribute ofnon-junctional E-cadherin, which inthe absence of cytoskeletal anchorage




has some leeway that allows theassociation with EGF-R in signalingplatforms known as lipid rafts.

As all investigations to date wereconducted with cultured cells, it can-not currently be judged whetherE-cadherin signaling is of relevancefor growth arrest in vivo. Indirectsupport is provided, as already men-tioned, by E-cadherin gene deletion,which results in some hyperprolifera-tion in the epidermis (Young et al.,2003; Tinkle et al., 2004; Tunggalet al., 2005). Furthermore, gene abla-tion of a-catenin in the epidermisresulted in a severe phenotype resem-bling pre-neoplastic transformations(Vasioukhin et al., 2001), whereas,unexpectedly, p120cnt knockout didnot impact tissue homeostasis if inflam-mation was prevented (Perez-Morenoet al., 2006). The a-catenin gene deletioncorrelated with ultrastructurally disturbedadherens junctions as well as reduceddesmosomes and tight junctions.In contrast, only adherens junctionswere affected in the case of the p120cnt

knockout. Collectively, these findingsmay suggest that if E-cadherin-mediatedgrowth inhibition and onset of terminaldifferentiation cannot occur in the epi-dermis in vivo, it can be compensated byother cadherins, including desmosomalcadherins and, as discussed below,possibly by Dsg3.

Desmosomal cadherin-mediated antipro-liferative events

Dsg3-mediated antiproliferative events.Anti-Dsg3 adhesion-disrupting anti-bodies are produced in the majority ofPemphigus vulgaris patients (Amagaiet al., 1991; Payne et al., 2004) orexperimentally in the mouse (mono-clonal anti-Dsg3 (AK23)) (Tsunodaet al., 2003). When added to culturedmouse keratinocytes at the beginningof the transitional phase, they induce,besides other events (for review seeAmagai et al., 2006), sustained prolif-eration, in correlation with a dramaticupregulation of c-Myc (Williamsonet al., 2006). Moreover, these antibo-dies disrupt the desmosomal organiza-tion, but initially do not disrupt theoranization of adherens junctions(de Bruin et al., 2007).

In agreement with the in vitro data,high c-Myc expression, sustained pro-liferation (in terms of increased Ki67-positive cells), and consistently, transe-pidermal keratin-14 expression wasobserved as a general feature of epi-dermis and oral mucosa in all humanand canine Pemphigus vulgaris patientsanalyzed so far ((Williamson et al.,2006, 2007a, b; de Bruin et al., 2007;Muller et al., 2007) and Lietti et al.,unpublished observations). Discoveredonly recently, the fact that sustainedproliferation does not result in hyper-plasia in the patient’s tissue mightexplain why this phenomenon has notbeen recognized previously. Hyperpro-liferation without hyperplasia can forinstance result from more rapid out-ward migration of basal epidermalkeratinocytes, as was observed inkeratin-10-knockout mice in presenceof increased c-Myc expression (Reich-elt and Magin, 2002).

In analogy to results on E-cadherin(for example, Calautti et al., 2005;Perrais et al., 2007), the phenotype ofsustained proliferation induced by anti-Dsg3 antibodies suggested a link be-tween transadhering Dsg3 and cell-cycle control. Recent data on Dsg3-mediated signaling in keratinocytes arein agreement with such a possibility(Figure 3, left panel).

At the beginning of the transitionalphase, desmocollin 3 and Dsg3 are themain desmosomal cadherins displayedat the cell surface of keratinocytes(Green and Gaudry, 2000; Duseket al., 2007). Similar to E-cadherin,they are stabilized following RhoAactivation (Waschke et al., 2006),Dsg3-associated plakoglobin becomestyrosine phosphorylated in a Src-familykinase-dependent manner (Calauttiet al., 1998) and PI3K (p85) associateswith the Dsg3/plakoglobin complex(SID Meeting abstract; Schulze et al.,2007). The formation of a Dsg3/PI3Kcomplex is followed by an increase inAkt activity and growth arrest (Liettiet al., unpublished observation) (Fig-ure 3, left panel). If cultured keratino-cytes are challenged at that stage withantibodies from Pemphigus vulgarispatients, they in contrast exhibit (withregards to Dsg3-mediated signaling) (a)reduced RhoA activity (demonstrated

in Pemphigus vulgaris antibody-treatedorganotypic cultures; Waschke et al.,2006), (b) enhanced turnover of mem-brane-soluble, non-junctional Dsg3(Calkins et al., 2006; Williamsonet al., 2006), and plasma membrane-anchored non-junctional Dsg3-asso-ciated plakoglobin (Williamson et al.,2006), (c) decreased non-junctionalDsg3 protein levels (Aoyama and Kita-jima, 1999; Calkins et al., 2006;Williamson et al., 2006; Yamamotoet al., 2007), (d) reduced nuclearimport of plakoglobin (Williamsonet al., 2006), (e) abrogation of plako-globin-mediated transcriptional repres-sion of c-Myc, and (e) sustainedproliferation concurrent with reducedexpression of differentiation markers(Figure 3, right panel) (Williamsonet al., 2006, 2007a, b). Furthermore,inhibition of both GSK3 and c-Mycprevents loss of intercellular adhesionin neonatal mice injected with Pem-phigus vulgaris antibodies or anti-Dsg3mouse monoclonal antibodies AK23.This firstly provides circumstantial evi-dence that GSK3 remains activated inpresence of pathogenic anti-Dsg3 anti-bodies and secondly underscores thepathogenic nature of this event (Wil-liamson et al., 2006; Figure 3).

Taken collectively, the signalingpathways downstream of transadheringDsg3 and changes upon binding ofPemphigus vulgaris antibody or AK23are reminiscent of events triggered bytransadhering E-cadherins. They sug-gest Dsg3-mediated activation of thePI3K/Akt pathway and growth arrestcontrasting with disruption of the PI3K/Akt pathway and continuing prolifera-tion at the expense of terminal differ-entiation in presence of anti-Dsg3antibodies (compare Figures 2 and 3).At that stage, Pemphigus vulgaris anti-bodies neither visibly affect the locali-zation or turn-over in individualcomponents of adherens junctions,nor do they prevent the nuclear importof b-catenin (Calkins et al., 2006;Williamson et al., 2006; de Bruinet al., 2007; Muller et al., 2007). Thissuggests that Dsg3 engagement issynergistic with and possibly amplifiesE-cadherin-mediated signaling to in-duce growth arrest via PI3K/Akt/GSK3and potentially EGF-R.



In support of a role of Dsg3 ingrowth control in vivo, a twofoldincrease in the proliferative index wasnoted in terms of Ki67 staining in thebasal layer of Dsg3-knockout mice (ourunpublished observation, Koch et al.,1997). Consistently, expression of atruncated Dsg3 transgene in basalepidermal keratinocytes, which lacksthe transadhesion domain, induced ahyperproliferative phenotype (Allenet al., 1996). Moreover, suprabasalmis-expression of full-length mouseDsg3 under the control of an involucrinpromoter disrupted the barrier functionbut failed to induce hyperproliferation,which is expected if Dsg3 has anti-proliferative activity (Elias et al., 2001).In contrast, human Dsg3 transgeneexpression driven by the keratin-1promoter displayed a hyperplastic phe-notype (Merritt et al., 2002). The exactreason for this discrepancy is currentlynot known.

Does Dsg3 control EGF-R signaling?.Indirect evidence for a control of Dsg3over EGF-R has been provided byactivation of EGF-R sustained inPemphigus vulgaris antibody-treatedkeratinocytes and A431 epithelial cells.

Upon exposure to Pemphigus vulgarisantibodies, EGF-R becomes rapidlyhyperphosphorylated, and in additionto Src, all three MAPK modules, ERK1/2and c-Jun, followed by p38, areactivated, (Sanchez-Carpintero et al.,2004; Berkowitz et al., 2005; Frusic-Zlotkin et al., 2006; Chernyavsky et al.,2007; Figure 3). Simultaneously, EGF-Rwas found to be internalized (Frusic-Zlotkin et al., 2006), reminiscent ofvascular endothelial growth factor re-ceptor 2-mediated signaling from in-tracellular stores (Lampugnani et al.,2006). These collective signalingevents involving EGF-R are character-istic of proliferating cells, which corre-lates with increased proliferationobserved in Pemphigus vulgaris anti-body-challenged mouse keratinocytesas well as in the patient (Williamsonet al., 2006, 2007a, b). Alternatively,Pemphigus vulgaris antibodies can alsoactivate apoptotic pathways in culturedkeratinocytes, as is supported by thepresence of apoptotic keratinocytes inthe blister roof of Pemphigus vulgarispatients (Frusic-Zlotkin et al.,2005, 2006; Chernyavsky et al.,2007). Induction of apoptotic effectorssubsequent to inhibition of PI3K/Akt is

expected, but only if EGF-R is blocked,that is in growth-arrested keratinocytes(Adams and Watt, 1989; Watt et al.,1993; Rodeck et al., 1997; Calauttiet al., 2005; Nikolopoulos et al., 2005).Such a correlation can for instance beobserved in cultured human keratino-cytes challenged with Pemphigusvulgaris antibodies (Chernyavskyet al., 2007).

In agreement with sustained EGF-Ractivation, basal keratinocytes in thePemphigus vulgaris patient’s epidermisas well as in culture ultimately showacantholysis or loss of intercellularadhesion (Payne et al., 2004). This isa known phenomenon in epithelialcells treated with EGF and was termed‘‘cell rounding’’ (Peppelenbosch et al.,1993). Mechanistically, EGF-inducedmodulation of intercellular adhesionwas suggested to rely on EGF-depen-dent tyrosine phosphorylation of pla-koglobin that compromises the linkbetween desmosomal cadherins andthe intermediate filament cytoskeleton(Gaudry et al., 2001). This appears tocorrespond with the ‘‘keratin retrac-tion’’ described in human Pemphigusvulgaris antibody-treated mouse kerati-nocytes (Jones et al., 1984; Caldelari

PG PG

Akt

PG

PGPG

Nucleus Nucleus

c-Jun

Control keratinocytes PV IgG-exposed keratinocytes

Dsg3 Dsg3 Dsg3

EGF-R EGF-R

ERK1/2 p38

c-Myc

Src

PGPG

c-Myc

Intercellular space

Plasma membrane

Pph Pph

PI3K

TyrRhoA

Src-kinase

PI3K

Tyr

Figure 3. Dsg3-mediated signaling pathways discussed to be activated during the transitional phase, under control conditions or in keratinocytes exposed to

Pemphigus vulgaris antibodies. Note that increased proliferation and c-Myc accumulation is observed in epidermis and oral mucosa of all human and

canine patients analyzed so far (Williamson et al., 2006, 2007a, b). Yellow stars, constitutively activated proteins. Note that the association of Dsg3/EGF-R

in control cells as well as the disrupted association between PG and DP of fully assembled desmosomes is based on circumstantial evidence, as is

constitutive activation of GSK3 in Pemphigus vulgaris-exposed keratinocytes. Results on Dsg3-mediated activation of the PI3K/Akt axis upstream of PG are

unpublished (Lietti et al., Schulze et al., unpublished observations). For abbreviations see Figure 2. DP, desmoplakin.




et al., 2001), as well as with theplakoglobin-mediated disruption ofthe desmosomal plaque (Caldelariet al., 2001; de Bruin et al., 2007)(Figure 3; right panel).

Cell rounding in EGF-treated HaCaTkeratinocytes can be prevented simplyby application of GSK3 inhibitors(Koivisto et al., 2006), exactly as wasshown in neonatal mice exposed toPemphigus vulgaris antibodies or AK23(Williamson et al., 2006). This under-scores that cell rounding relies onGSK3 activation related to EGF-R sig-naling. Hence, these events are con-secutive and interdependent, withGSK3 being upstream of EGF-R (Fig-ure 3). This would agree with thefinding that c-Myc gene transcription(downstream of GSK3b) is required topropagate Ras/MAPK induced uncon-trolled proliferation (Oskarsson et al.,2006), whereby Ras activates theMAPK pathways downstream of EGF-R. Further supporting the GSK3–EGF-Rhierarchy, plakoglobin-knockout kera-tinocytes do not undergo keratin retrac-tion in response to Pemphigus vulgarisantibodies, despite elevated c-Myclevels (Caldelari et al., 2001; William-son et al., 2006), because plakoglobinwas found to integrate EGF-R-mediatedsignaling in mouse keratinocytes interms of loss of keratin anchorage(Gaudry et al., 2001). Furthermore,c-Myc inhibition alone is sufficient toprevent acantholysis (without inhibitingEGF-R) (Williamson et al., 2006). Fi-nally, the fact that only basal keratino-cytes are responsive to Pemphigusvulgaris antibodies in terms of ‘‘cellrounding’’ could relate to the findingthat Ras activation downstream of EGF-Ris dependent on cell–substrate adhesion(Mainiero et al., 1997) and hence isswitched off in suprabasal keratino-cytes (Dajee et al., 2002).

Is Dsg3 signaling triggered by non-junctional Dsg3? Pathogenic Dsg3antibodies target the N-terminal do-main (patients or mouse monoclonal(AK23); Tsunoda et al., 2003) andinterfere with growth arrest. E-cadherinantibodies that bind to the N-terminaldomain, disrupt E-cadherin engage-ment (Hoschuetzky et al., 1994) andprevent PI3K/Akt activation (Pece et al.,

1999; Calautti et al., 2005). Due to thesimilarity of the evoked signaling path-ways, pathogenic Dsg3 antibodies mayalso exert their activity by disruptingtransadhesion between Dsg3 mole-cules. This mechanism was addressedas ‘‘steric hindrance’’ (Futei et al.,2000; Tsunoda et al., 2003). Severallines of evidence suggest that Dsg3antibodies further activate cell signal-ing by binding to transadhering non-junctional Dsg3, like discussed here forintegrin and E-cadherin receptors; (a)non-keratin-anchored Dsg3 is the pri-mary target of Pemphigus vulgarisantibodies observed in time-lapse stu-dies in vitro (Sato et al., 2000), (b)Pemphigus vulgaris antibodies bind tothe interdesmosomal space as observedin Pemphigus vulgaris patients biopsies(Suter et al., 1990; Bedane et al., 1996),(c) split formation between epidermalkeratinocytes in mice injected withPemphigus vulgaris antibodies firstoccurs between desmosomes (Takaha-shi et al., 1985), and (d) in contrast tonon-junctional Dsg3, the desmosomalfraction is not affected at the timechanges in diverse effectors includingnuclear targeting of plakoglobin aremeasured (Williamson et al., 2006).Dsg3 in the desmosomal fraction isreduced at later time points, whichlikely represents an important eventpreceding acantholysis (Koch et al.,1997; Aoyama and Kitajima, 1999;Williamson et al., 2006; Yamamotoet al., 2007).

Conclusion on Dsg3/EGF-R signalingin epidermal keratinocytes. Reminis-cent of studies on E-cadherin, investi-gations on Dsg3 highlight its capabilityto activate signaling cascades involvingPI3K/Akt and potentially EGF-R, toexert growth control in vitro andin vivo. Although still limited, the currentview on Dsg3-mediated signaling issupported by the consequences of anti-Dsg3 antibodies on cell growth seen inPemphigus vulgaris patients.

The emerging concept is thatanti-Dsg3 antibodies oppose Dsg3-mediated signaling by disruptingtransadhesion between non-junctionalDsg3 through steric hindrance. Thisprovides a signal of ‘‘no cell’’ contact,GSK3 and EGF-R fail to be inhibited,

and the keratinocytes in the basal layerof the epidermis continue to proliferate.As a consequence, the desmosomalanchorage is reduced through aplakoglobin/EGF-R-mediated process(Figure 3), Dsg3 is depleted fromdesmosomes, and adhesiveness isweakened resulting in the formation ofblisters. Noticeable, hypotheses otherthan the one discussed here have beenraised for Pemphigus vulgaris patho-genesis. They do however not addressDsg3-mediated growth control, and havebeen discussed in detail elsewhere (forreview see Amagai et al., 2006).

With respect to the detailedmechanism of Dsg3-mediated signaling,some aspects described here are con-firmed, some are based on circumstan-tial evidence, while many others suchas Akt inhibition in Pemphigus vulgarischallenged keratinocytes, disruption bysteric hindrance, and the relationshipGSK3–EGF-R need to be filled in.Nonetheless, these collective resultsassembled with respect to Dsg3 raisethe possibility that non-junctional Dsg3represents an extraordinary sensorysystem of basal keratinocytes in theway suggested for E-cadherin.

The relative importance of E-cadher-in, Dsg3, and other desmosomal cad-herins in mediating antiproliferativesignals in basal epidermal keratino-cytes requires further detailed studies.Importantly, however, E-cadherindistribution and b-catenin nucleartranslocation are not initially affectedby Pemphigus vulgaris antibodies(Williamson et al., 2006; de Bruinet al., 2007; Muller et al., 2007),suggesting that the pathways down-stream of E-cadherin and Dsg3 areseparable, as could also be suggestedby the results on PI3K recruitment tothe E-cadherin tail (Calautti et al.,2005; Xie and Bikle, 2007).

Conclusion and outlook

Epidermal homeostasis requires a bal-ance of pro- and antiproliferative sig-nals in addition to survival signals(Figures 2 and 3). The current knowl-edge on adhesion receptor signalingsummarized herein highlights the cri-tical control the integrin and cadherinreceptor network in cooperation withEGF-R (or other RTK) can impose on



most of these crucial signaling events. Itis further proposed that the adhesionreceptors exert their control by com-municating with the cell’s immediateenvironment in absence of their cytos-keletal anchorage. This would facilitatethe association with EGF-R in lipidrafts. Although we are far from under-standing the complex interplay bet-ween the entire adhesive network,the fact that experimental disruptionof adhesion or mimicry of adhesion issufficient to change cell fate, stronglysuggests that adhesion receptors have afundamental role in regulating impor-tant homeostatic processes also in vivo.Consistent with such a scenario, con-stitutive activation or disruption ofthese receptor functions in vivo, in-cluding disturbed expression of adhe-sion components, leads to uncontrolledproliferation, resulting in severe pathol-ogies such as cancer, psoriasis, and assuggested more recently, also Pemphi-gus vulgaris. Further investigations ofthe interdependent signaling pathwaysbetween integrins, cadherins, and RTKduring epidermal renewal will contri-bute to a better understanding of theircomplex interplay and provide novelinsights into the mechanisms under-lying these severe pathologies.

CONFLICT OF INTERESTThe authors state no conflict of interest.

ACKNOWLEDGMENTSWe are most grateful to Professor K Greenand Dr S Getsios, Departments of Pathology andDermatology, Northwestern University, Chicago,Professor T Hunziker, Department of Dermato-logy, University of Bern, Switzerland and Dr E.Calautti, MBC, University of Turin, Italy, forhelpful comments on the paper, as well as PeterGirling, CELLnTEC advanced cell systems, Bern,Switzerland, for editorial support. The work fromour laboratory was supported by the MarthaStiftung (Zurich) and the Swiss National ScienceFoundation (Grant 3100A0-107243).

REFERENCES

Aberle H, Bauer A, Stappert J, Kispert A, Kemler R(1997) Beta-catenin is a target for the

ubiquitin–proteasome pathway. EMBO J

16:3797–804

Adams JC, Watt FM (1989) Fibronectin inhibitsthe terminal differentiation of human kerati-

nocytes. Nature 340:307–9

Allen E, Yu QC, Fuchs E (1996) Mice expressing amutant desmosomal cadherin exhibit ab-

normalities in desmosomes, proliferation,

and epidermal differentiation. J Cell Biol133:1367–82

Alt A, Gartsbein M, Ohba M, Kuroki T, Tennen-baum T (2004) Differential regulation ofalpha6beta4 integrin by PKC isoforms inmurine skin keratinocytes. Biochem BiophysRes Commun 314:17–23

Amagai M, Ahmed AR, Kitajima Y, Bystryn JC,Milner Y, Gniadecki R et al. (2006) Aredesmoglein autoantibodies essential for theimmunopathogenesis of pemphigus vulgaris,or just ‘‘witnesses of disease’’? Exp Dermatol15:815–31

Amagai M, Klaus-Kovtun V, Stanley JR (1991)Autoantibodies against a novel epithelialcadherin in pemphigus vulgaris, a diseaseof cell adhesion. Cell 67:869–77

Anastasiadis PZ (2007) p120-ctn: a nexusfor contextual signaling via Rho GTPases.Biochim Biophys Acta 1773:34–46

Aoyama Y, Kitajima Y (1999) Pemphigus vulgaris-IgG causes a rapid depletion of desmoglein 3(Dsg3) from the Triton X-100 soluble pools,leading to the formation of Dsg3-depleteddesmosomes in a human squamous carcino-ma cell line, DJM-1 cells. J Invest Dermatol112:67–71

Arnold I, Watt FM (2001) c-Myc activation intransgenic mouse epidermis results in mobi-lization of stem cells and differentiation oftheir progeny. Curr Biol 11:558–68

Baker J, Garrod D (1993) Epithelial cells retainjunctions during mitosis. J Cell Sci 104(Part2):415–25

Bauer A, Lickert H, Kemler R, Stappert J (1998)Modification of the E-cadherin–catenin com-plex in mitotic Madin–Darby canine kidneyepithelial cells. J Biol Chem 273:28314–21

Bedane C, Prost C, Thomine E, Intrator L, Joly P,Caux F et al. (1996) Binding of autoantibo-dies is not restricted to desmosomes inpemphigus vulgaris: comparison of 14 casesof pemphigus vulgaris and 10 cases ofpemphigus foliaceus studied by westernimmunoblot and immunoelectron micro-scopy. Arch Dermatol Res 288:343–52

Berkowitz P, Hu P, Liu Z, Diaz LA, Enghild JJ,Chua MP et al. (2005) Desmosome signaling.Inhibition of p38MAPK prevents pemphigusvulgaris IgG-induced cytoskeleton reorgani-zation. J Biol Chem 280:23778–84

Blanpain C, Fuchs E (2006) Epidermal stem cellsof the skin. Annu Rev Cell Dev Biol 22:339–73

Boller K, Vestweber D, Kemler R (1985) Cell-adhesion molecule uvomorulin is localizedin the intermediate junctions of adult in-testinal epithelial cells. J Cell Biol 100:327–32

Butz S, Stappert J, Weissig H, Kemler R (1992)Plakoglobin and beta-catenin: distinct butclosely related. Science 257:1142–4

Cabodi S, Calautti E, Talora C, Kuroki T, Stein PL,Dotto GP (2000) A PKC-eta/Fyn-dependentpathway leading to keratinocyte growtharrest and differentiation. Mol Cell 6:1121–9

Calautti E, Cabodi S, Stein PL, Hatzfeld M,Kedersha N, Paolo Dotto G (1998) Tyrosine

phosphorylation and src family kinases con-trol keratinocyte cell-cell adhesion. J CellBiol 141:1449–65

Calautti E, Grossi M, Mammucari C, Aoyama Y,Pirro M, Ono Y et al. (2002) Fyn tyrosinekinase is a downstream mediator of Rho/PRK2 function in keratinocyte cell-cell ad-hesion. J Cell Biol 156:137–48

Calautti E, Li J, Saoncella S, Brissette JL, GoetinckPF (2005) Phosphoinositide 3-kinase signal-ing to Akt promotes keratinocyte differentia-tion versus death. J Biol Chem 280:32856–65

Caldelari R, de Bruin A, Baumann D, Suter MM,Bierkamp C, Balmer V et al. (2001) A centralrole for the armadillo protein plakoglobin inthe autoimmune disease pemphigus vulgaris.J Cell Biol 153:823–34

Calkins CC, Setzer SV, Jennings JM, Summers S,Tsunoda K, Amagai M et al. (2006) Desmo-glein endocytosis and desmosome disassem-bly are coordinated responses to pemphigusautoantibodies. J Biol Chem 281:7623–34

Carmeliet P, Lampugnani MG, Moons L, BreviarioF, Compernolle V, Bono F et al. (1999)Targeted deficiency or cytosolic truncationof the VE-cadherin gene in mice impairsVEGF-mediated endothelial survival andangiogenesis. Cell 98:147–57

Chan KS, Carbajal S, Kiguchi K, Clifford J, Sano S,DiGiovanni J (2004) Epidermal growth factorreceptor-mediated activation of Stat3 duringmultistage skin carcinogenesis. Cancer Res64:2382–9

Chernyavsky AI, Arredondo J, Kitajima Y, Sato-Nagai M, Grando SA (2007) Desmoglein vsnon-desmoglein signaling in pemphigusacantholysis: characterization of novelsignaling pathways downstream of pemphi-gus vulgaris antigens. J Biol Chem 282:13804–12

Cowan KJ, Storey KB (2003) Mitogen-activatedprotein kinases: new signaling pathwaysfunctioning in cellular responses to environ-mental stress. J Exp Biol 206:1107–15

Dajee M, Lazarov M, Zhang JY, Cai T, Green CL,Russell AJ et al. (2003) NF-kappaB blockadeand oncogenic Ras trigger invasive humanepidermal neoplasia. Nature 421:639–43

Dajee M, Tarutani M, Deng H, Cai T, Khavari PA(2002) Epidermal Ras blockade demonstratesspatially localized Ras promotion of prolif-eration and inhibition of differentiation.Oncogene 21:1527–38

de Bruin A, Caldelari R, Williamson L, Suter MM,Hunziker T, Wyder M et al. (2007) Plakoglo-bin-dependent disruption of the desmosomalplaque in pemphigus vulgaris. Exp Dermatol16:468–75

Dejana E (2004) Endothelial cell–cell junctions:happy together. Nat Rev Mol Cell Biol5:261–70

Denning MF, Dlugosz AA, Williams EK,Szallasi Z, Blumberg PM, Yuspa SH (1995)Specific protein kinase C isozymes mediatethe induction of keratinocyte differentiationmarkers by calcium. Cell Growth Differ6:149–57

Dusek RL, Godsel LM, Green KJ (2007) Discrimi-nating roles of desmosomal cadherins:




beyond desmosomal adhesion. J DermatolSci 45:7–21

Elias PM, Matsuyoshi N, Wu H, Lin C, Wang ZH,Brown BE et al. (2001) Desmoglein isoformdistribution affects stratum corneum struc-ture and function. J Cell Biol 153:243–9

Engelman JA, Luo J, Cantley LC (2006) Theevolution of phosphatidylinositol 3-kinasesas regulators of growth and metabolism. NatRev Genet 7:606–19

Espada J, Perez-Moreno M, Braga VMM, Rodri-guez-Viciana P, Cano A (1999) H-Rasactivation promotes cytoplasmic accumula-tion and phosphoinositide 3-OH kinaseassociation of {beta}-catenin in epidermalkeratinocytes. J Cell Biol 146:967–80

Frisch SM, Screaton RA (2001) Anoikis mechan-isms. Curr Opin Cell Biol 13:555–62

Frusic-Zlotkin M, Pergamentz R, Michel B, DavidM, Mimouni D, Bregegere F et al. (2005) Theinteraction of pemphigus autoimmunoglobu-lins with epidermal cells: activation of the fasapoptotic pathway and the use of caspaseactivity for pathogenicity tests of pemphiguspatients. Ann N Y Acad Sci 1050:371–9

Frusic-Zlotkin M, Raichenberg D, Wang X, DavidM, Michel B, Milner Y (2006) Apoptoticmechanism in pemphigus autoimmunoglo-bulins—induced acantholysis-possible invol-vement of the EGF receptor. Autoimmunity39:563–75

Fuchs M, Muller T, Lerch MM, Ullrich A (1996)Association of human protein-tyrosine phos-phatase kappa with members of the arma-dillo family. J Biol Chem 271:16712–9

Futei Y, Amagai M, Sekiguchi M, Nishifuji K, FujiiY, Nishikawa T (2000) Use of domain-swapped molecules for conformational epi-tope mapping of desmoglein 3 in pemphigusvulgaris. J Invest Dermatol 115:829–34

Gagnoux-Palacios L, Dans M, van’t Hof W,Mariotti A, Pepe A, Meneguzzi G et al.(2003) Compartmentalization of integrinalpha6beta4 signaling in lipid rafts. J CellBiol 162:1189–96

Gambardella L, Barrandon Y (2003) The multi-faceted adult epidermal stem cell. Curr OpinCell Biol 15:771–7

Gaudry CA, Palka HL, Dusek RL, Huen AC,Khandekar MJ, Hudson LG et al. (2001)Tyrosine-phosphorylated plakoglobinis associated with desmogleins but notdesmoplakin after epidermal growth factorreceptor activation. J Biol Chem 276:24871–80

Green H (1977) Terminal differentiation ofcultured human epidermal cells. Cell11:405–16

Green KJ, Gaudry CA (2000) Are desmosomesmore than tethers for intermediate filaments?Nat Rev Mol Cell Biol 1:208–16

Gumbiner BM (2000) Regulation of cadherinadhesive activity. J Cell Biol 148:399–404

Guo W, Giancotti FG (2004) Integrin signallingduring tumour progression. Nat Rev Mol CellBiol 5:816–26

Guo W, Pylayeva Y, Pepe A, Yoshioka T, MullerWJ, Inghirami G et al. (2006) Beta 4 integrin

amplifies ErbB2 signaling to promote mam-mary tumorigenesis. Cell 126:489–502

Hofmann C, Obermeier F, Artinger M, HausmannM, Falk W, Schoelmerich J et al. (2007)Cell–cell contacts prevent anoikis in primaryhuman colonic epithelial cells. Gastroenter-ology 132:587–600

Hoschuetzky H, Aberle H, Kemler R (1994) Beta-catenin mediates the interaction of thecadherin–catenin complex with epidermalgrowth factor receptor. J Cell Biol 127:1375–80

Ishizawar R, Parsons SJ (2004) c-Src and co-operating partners in human cancer. CancerCell 6:209–14

Janes SM, Watt FM (2006) New roles for integrinsin squamous-cell carcinoma. Nat Rev Can-cer 6:175–83

Jones JCR, Arnn J, Staehelin LA, Goldman RD(1984) Human autoantibodies against des-mosomes: possible causative factors in pem-phigus. Proc Natl Acad Sci USA 81:2781–5

Kang HG, Jenabi JM, Zhang J, Keshelava N,Shimada H, May WA et al. (2007) E-cadherincell–cell adhesion in Ewing tumor cellsmediates suppression of anoikis throughactivation of the ErbB4 tyrosine kinase.Cancer Res 67:3094–105

Koch PJ, Mahoney MG, Ishikawa H, Pulkkinen L,Uitto J, Shultz L et al. (1997) Targeteddisruption of the pemphigus vulgaris antigen(desmoglein 3) gene in mice causes loss ofkeratinocyte cell adhesion with a phenotypesimilar to pemphigus vulgaris. J Cell Biol137:1091–102

Kodama S, Ikeda S, Asahara T, Kishida M, KikuchiA (1999) Axin directly interacts with plako-globin and regulates its stability. J Biol Chem274:27682–8

Koivisto L, Jiang G, Hakkinen L, Chan B, LarjavaH (2006) HaCaT keratinocyte migration isdependent on epidermal growth factor re-ceptor signaling and glycogen synthasekinase-3alpha. Exp Cell Res 312:2791–805

Kolly C, Suter MM, Muller EJ (2005) Proliferation,cell cycle exit, and onset of terminaldifferentiation in cultured keratinocytes:pre-programmed pathways in control of C-Myc and Notch1 prevail over extracellularcalcium signals. J Invest Dermatol 124:1014–25

Kovacs EM, Ali RG, McCormack AJ, Yap AS(2002) E-cadherin homophilic ligation di-rectly signals through Rac and phosphatidy-linositol 3-kinase to regulate adhesivecontacts. J Biol Chem 277:6708–18

Kusumi A, Sako Y, Yamamoto M (1993) Confinedlateral diffusion of membrane receptors asstudied by single particle tracking (nanovidmicroscopy). Effects of calcium-induceddifferentiation in cultured epithelial cells.Biophys J 65:2021–40

Lampugnani MG, Orsenigo F, Gagliani MC,Tacchetti C, Dejana E (2006) Vascularendothelial cadherin controls VEGFR-2 in-ternalization and signaling from intracellularcompartments. J Cell Biol 174:593–604

Laprise P, Chailler P, Houde M, Beaulieu JF,Boucher MJ, Rivard N (2002) Phosphatidyli-

nositol 3-kinase controls human intestinalepithelial cell differentiation by promotingadherens junction assembly and p38 MAPKactivation. J Biol Chem 277:8226–34

Laprise P, Langlois MJ, Boucher MJ, Jobin C,Rivard N (2004) Down-regulation of MEK/ERK signaling by E-cadherin-dependentPI3K/Akt pathway in differentiating intestinalepithelial cells. J Cell Physiol 199:32–9

Lewis JE, Wahl JK III, Sass KM, Jensen PJ, JohnsonKR, Wheelock MJ (1997) Cross-talk betweenadherens junctions and desmosomes de-pends on plakoglobin. J Cell Biol 136:919–34

Li W, Marshall C, Mei L, Gelfand J, Seykora JT(2007) Srcasm corrects Fyn-induced epider-mal hyperplasia by kinase downregulation.J Biol Chem 282:1161–9

Liebner S, Cavallaro U, Dejana E (2006) Themultiple languages of endothelial cell-to-cellcommunication. Arterioscler Thromb VascBiol 26:1431–8

Lopez-Rovira T, Silva-Vargas V, Watt FM (2005)Different consequences of beta1 integrindeletion in neonatal and adult mouse epi-dermis reveal a context-dependent role ofintegrins in regulating proliferation, differen-tiation, and intercellular communication.J Invest Dermatol 125:1215–27

Mainiero F, Murgia C, Wary KK, Curatola AM,Pepe A, Blumemberg M et al. (1997) Thecoupling of alpha6beta4 integrin to Ras–MAP kinase pathways mediated by Shccontrols keratinocyte proliferation. EMBO J16:2365–75

Mainiero F, Pepe A, Yeon M, Ren Y, Giancotti FG(1996) The intracellular functions of alpha6-beta4 integrin are regulated by EGF. J CellBiol 134:241–53

Marcozzi C, Burdett ID, Buxton RS, Magee AI(1998) Coexpression of both types of desmo-somal cadherin and plakoglobin confersstrong intercellular adhesion. J Cell Sci111:495–509

Mariotti A, Kedeshian PA, Dans M, Curatola AM,Gagnoux-Palacios L, Giancotti FG (2001)EGF-R signaling through Fyn kinase disruptsthe function of integrin alpha6beta4 athemidesmosomes: role in epithelial cellmigration and carcinoma invasion. J CellBiol 155:447–58

McLachlan RW, Kraemer A, Helwani FM, KovacsEM, Yap AS (2007) E-cadherin adhesionactivates c-Src signaling at cell–cell contacts.Mol Biol Cell 18:3214–23

Merritt AJ, Berika MY, Zhai W, Kirk SE, Ji B,Hardman MJ et al. (2002) Suprabasal des-moglein 3 expression in the epidermis oftransgenic mice results in hyperproliferationand abnormal differentiation. Mol Cell Biol22:5846–58

Miettinen PJ, Berger JE, Meneses J, Phung Y,Pedersen RA, Werb Z et al. (1995) Epithelialimmaturity and multiorgan failure in micelacking epidermal growth factor receptor.Nature 376:337–41

Miranti CK, Brugge JS (2002) Sensing the envir-onment: a historical perspective on integrinsignal transduction. Nat Cell Biol 4:E83–90



Missero C, Di Cunto F, Kiyokawa H, Koff A, DottoGP (1996) The absence of p21Cip1/WAF1alters keratinocyte growth and differentiationand promotes ras-tumor progression. GenesDev 10:3065–75

Muller EJ, Hunziker T, Suter MM (2007) Keratinintermediate filament retraction is linked toplakoglobin-dependent signaling in pemphi-gus vulgaris. J Am Acad Dermatol 56:890–1

Nikolopoulos SN, Blaikie P, Yoshioka T, Guo W,Puri C, Tacchetti C et al. (2005) Targeteddeletion of the integrin beta4 signalingdomain suppresses laminin-5-dependent nu-clear entry of mitogen-activated proteinkinases and NF-kappaB, causing defects inepidermal growth and migration. Mol CellBiol 25:6090–102

Nishio H, Matsui K, Tsuji H, Tamura A, Suzuki K(2001) Immunolocalisation of the Januskinases (JAK)–signal transducers and activa-tors of transcription (STAT) pathway inhuman epidermis. J Anat 198:581–9

Noren NK, Niessen CM, Gumbiner BM, BurridgeK (2001) Cadherin engagement regulates rhofamily GTPases. J Biol Chem 276:33305–8

Oskarsson T, Essers MA, Dubois N, Offner S,Dubey C, Roger C et al. (2006) Skinepidermis lacking the c-Myc gene is resistantto Ras-driven tumorigenesis but can reac-quire sensitivity upon additional loss of thep21Cip1 gene. Genes Dev 20:2024–9

Owens DM, Romero MR, Gardner C, Watt FM(2003) Suprabasal {alpha}6{beta}4 integrinexpression in epidermis results in enhancedtumorigenesis and disruption of TGF{beta}signalling. J Cell Sci 116:3783–91

Pang J-H, Kraemer A, Stehbens SJ, Frame MC, YapAS (2005) Recruitment of phosphoinositide3-kinase defines a positive contribution oftyrosine kinase signaling to E-cadherin func-tion. J Biol Chem 280:3043–50

Pankow S, Bamberger C, Klippel A, Werner S(2006) Regulation of epidermal homeostasisand repair by phosphoinositide 3-kinase.J Cell Sci 119:4033–46

Payne AS, Hanakawa Y, Amagai M, Stanley JR(2004) Desmosomes and disease: pemphigusand bullous impetigo. Curr Opin Cell Biol16:536–43

Pece S, Chiariello M, Murga C, Gutkind JS (1999)Activation of the protein kinase Akt/PKB bythe formation of E-cadherin-mediated cell–-cell junctions. Evidence for the association ofphosphatidylinositol 3-kinase with the E-cadherin adhesion complex. J Biol Chem274:19347–51

Pece S, Gutkind JS (2000) Signaling from E-cadherins to the MAPK pathway by therecruitment and activation of epidermalgrowth factor receptors upon cell–cell con-tact formation. J Biol Chem 275:41227–33

Peng XD, Xu PZ, Chen ML, Hahn-Windgassen A,Skeen J, Jacobs J et al. (2003) Dwarfism,impaired skin development, skeletal muscleatrophy, delayed bone development, andimpeded adipogenesis in mice lacking Akt1and Akt2. Genes Dev 17:1352–65

Peppelenbosch MP, Tertoolen LG, Hage WJ,de Laat SW (1993) Epidermal growth

factor-induced actin remodeling is regulatedby 5-lipoxygenase and cyclooxygenase pro-ducts. Cell 74:565–75

Perez-Moreno M, Davis MA, Wong E, Pasolli HA,Reynolds AB, Fuchs E (2006) p120-cateninmediates inflammatory responses in the skin.Cell 124:631–44

Perrais M, Chen X, Perez-Moreno M, GumbinerBM (2007) E-cadherin homophilic ligationinhibits cell growth and epidermal growthfactor receptor signaling independent ofother cell interactions. Mol Biol Cell18:2013–25

Pierres A, Prakasam A, Touchard D, Benoliel AM,Bongrand P, Leckband D (2007) Dissectingsubsecond cadherin bound states reveals anefficient way for cells to achieve ultrafastprobing of their environment. FEBS Lett581:1841–6

Pillai S, Bikle DD, Mancianti ML, Cline P,Hincenbergs M (1990) Calcium regulationof growth and differentiation of normalhuman keratinocytes: modulation of differ-entiation competence by stages of growthand extracellular calcium. J Cell Physiol143:294–302

Poumay Y, Pittelkow MR (1995) Cell density andculture factors regulate keratinocyte commit-ment to differentiation and expression ofsuprabasal K1/K10 keratins. J Invest Derma-tol 104:271–6

Qian X, Karpova T, Sheppard AM, McNally J,Lowy DR (2004) E-cadherin-mediated adhe-sion inhibits ligand-dependent activation ofdiverse receptor tyrosine kinases. EMBO J23:1739–48

Raghavan S, Vaezi A, Fuchs E (2003) A role foralphabeta1 integrins in focal adhesion func-tion and polarized cytoskeletal dynamics.Dev Cell 5:415–27

Raymond K, Kreft M, Janssen H, Calafat J,Sonnenberg A (2005) Keratinocytes displaynormal proliferation, survival and differentia-tion in conditional beta4-integrin knockoutmice. J Cell Sci 118:1045–60

Reichelt J, Magin TM (2002) Hyperproliferation,induction of c-Myc and 14-3-3sigma, but nocell fragility in keratin-10-null mice. J CellSci 115:2639–50

Rodeck U, Jost M, Kari C, Shih DT, Lavker RM,Ewert DL et al. (1997) EGF-R dependentregulation of keratinocyte survival. J Cell Sci110(Part 2):113–21

Roh JY, Stanley JR (1995) Intracellular domain ofdesmoglein 3 (pemphigus vulgaris antigen)confers adhesive function on the extracellu-lar domain of E-cadherin without bindingcatenins. J Cell Biol 128:939–47

Sabapathy K, Wagner EF (2004) JNK2: a negativeregulator of cellular proliferation. Cell Cycle3:1520–3

Sanchez-Carpintero I, Espana A, Pelacho B, LopezMoratalla N, Rubenstein DS, Diaz LA et al.(2004) In vivo blockade of pemphigusvulgaris acantholysis by inhibition of intra-cellular signal transduction cascades. Br JDermatol 151:565–70

Sato M, Aoyama Y, Kitajima Y (2000) Assemblypathway of desmoglein 3 to desmosomes

and its perturbation by pemphigus vulgaris-IgG in cultured keratinocytes, as revealedby time-lapsed labeling immunoelectronmicroscopy. Lab Invest 80:1583–92

Schmidt E, Zillikens D (2000) Autoimmune andinherited subepidermal blistering diseases:advances in the clinic and the laboratory.Adv Dermatol 16:113–57

Schulze K, Lietti C, Strauss C, Williamson L,Suter MM, Muller EJ (2007) Pemphigusvulgaris identifies non-junctional desmoglein3 as regulator of tissue homeostasis viaplakglobin signaling. J Invest Dermatol 127:485

Seveau S, Bierne H, Giroux S, Prevost MC,Cossart P (2004) Role of lipid rafts inE-cadherin—and HGF-R/Met—mediatedentry of Listeria monocytogenes into hostcells. J Cell Biol 166:743–53

Shaw LM, Rabinovitz I, Wang HH, Toker A,Mercurio AM (1997) Activation of phosphoi-nositide 3-OH kinase by the alpha6beta4integrin promotes carcinoma invasion. Cell91:949–60

Stover DR, Becker M, Liebetanz J, Lydon NB(1995) Src phosphorylation of the epidermalgrowth factor receptor at novel sites med-iates receptor interaction with Src and P85alpha. J Biol Chem 270:15591–7

Suter MM, de Bruin A, Wyder M, Warm S,Credille K, Crameri FM et al. (1998) Auto-immune diseases of domestic animals: anupdate. Adv Vet Dermatol 3:321–37

Suter MM, Wilkinson JE, Dougherty EP, Lewis RM(1990) Ultrastructural localization ofpemphigus vulgaris antigen on caninekeratinocytes in vivo and in vitro. Am J VetRes 51:507–11

Takahashi K, Suzuki K (1996) Density-dependentinhibition of growth involves prevention ofEGF receptor activation by E-cadherin-mediated cell–cell adhesion. Exp Cell Res226:214–22

Takahashi Y, Patel HP, Labib RS, Diaz LA, AnhaltGJ (1985) Experimentally induced pemphi-gus vulgaris in neonatal BALB/c mice: atime-course study of clinical, immunologic,ultrastructural, and cytochemical changes.J Invest Dermatol 84:41–6

Takeda H, Nagafuchi A, Yonemura S, Tsukita S,Behrens J, Birchmeier W et al. (1995) V-srckinase shifts the cadherin-based cell adhe-sion from the strong to the weak state andbeta catenin is not required for the shift.J Cell Biol 131:1839–47

Theisen CS, Wahl JK III, Johnson KR, WheelockMJ (2007) NHERF links the N-cadherin/catenin complex to the PDGF receptor tomodulate the actin cytoskeleton and regulatecell motility. Mol Biol Cell 18:1220–32

Thiery JP (2002) Epithelial–mesenchymal transi-tions in tumour progression. Nat Rev Cancer2:442–54

Thrash BR, Menges CW, Pierce RH, McCance DJ(2006) AKT1 provides an essential survivalsignal required for differentiation and strati-fication of primary human keratinocytes.J Biol Chem 281:12155–62

Tinkle CL, Lechler T, Pasolli HA, Fuchs E (2004)Conditional targeting of E-cadherin in skin:




insights into hyperproliferative and degen-erative responses. Proc Natl Acad Sci USA101:552–7

Toker A, Yoeli-Lerner M (2006) Akt signaling andcancer: surviving but not moving on. CancerRes 66:3963–6

Tsunoda K, Ota T, Aoki M, Yamada T, Nagai T,Nakagawa T et al. (2003) Induction ofpemphigus phenotype by a mousemonoclonal antibody against the amino-terminal adhesive interface of desmoglein3. J Immunol 170:2170–8

Tunggal JA, Helfrich I, Schmitz A, Schwarz H,Gunzel D, Fromm M et al. (2005) E-cadherinis essential for in vivo epidermal barrierfunction by regulating tight junctions. EMBOJ 24:1146–56

Vasioukhin V, Bauer C, Degenstein L, Wise B,Fuchs E (2001) Hyperproliferation and de-fects in epithelial polarity upon conditionalablation of alpha-catenin in skin. Cell104:605–17

Vasioukhin V, Fuchs E (2001) Actin dynamics andcell–cell adhesion in epithelia. Curr OpinCell Biol 13:76–84

Vivanco I, Sawyers CL (2002) The phos-phatidylinositol 3-kinase AKT pathwayin human cancer. Nat Rev Cancer 2:489–501

Waikel RL, Kawachi Y, Waikel PA, Wang XJ,Roop DR (2001) Deregulated expression ofc-Myc depletes epidermal stem cells. NatGenet 28:165–8

Waschke J, Spindler V, Bruggeman P, Zillikens D,Schmidt G, Drenckhahn D (2006) Inhibitionof Rho A activity causes pemphigus skinblistering. J Cell Biol 175:721–7

Watt FM (2002) Role of integrins in regulatingepidermal adhesion, growth and differentia-tion. EMBO J 21:3919–26

Watt FM, Kubler MD, Hotchin NA, Nicholson LJ,Adams JC (1993) Regulation of keratinocyteterminal differentiation by integrin–extracellularmatrix interactions. J Cell Sci 106(Part 1):175–82

Watt FM, Lo Celso C, Silva-Vargas V (2006)Epidermal stem cells: an update. Curr OpinGenet Dev 16:518–24

Weston CR, Wong A, Hall JP, Goad ME, FlavellRA, Davis RJ (2004) The c-Jun NH2-terminalkinase is essential for epidermal growthfactor expression during epidermal morpho-genesis. Proc Natl Acad Sci USA101:14114–9

White DE, Kurpios NA, Zuo D, Hassell JA, BlaessS, Mueller U et al. (2004) Targeted disruptionof beta1-integrin in a transgenic mousemodel of human breast cancer reveals anessential role in mammary tumor induction.Cancer Cell 6:159–70

Wilhelmsen K, Litjens SH, Sonnenberg A (2006)Multiple functions of the integrin alpha6be-ta4 in epidermal homeostasis and tumori-genesis. Mol Cell Biol 26:2877–86

Williamson L, Hunziker T, Suter MM, Muller EJ(2007a) Nuclear c-Myc: a molecular markerfor early stage pemphigus vulgaris. J InvestDermatol 127:1549–55

Williamson L, Raess NA, Caldelari R, Zakher A,de Bruin A, Posthaus H et al. (2006)Pemphigus vulgaris identifies plakoglobinas key suppressor of c-Myc in the skin.EMBO J 25:3298–309

Williamson L, Suter MM, Olivry T, Wyder M,Muller EJ (2007b) Upregulation of c-Myc

may contribute to the pathogenesis of caninepemphigus vulgaris. Vet Dermatol 18:12–7

Woodfield RJ, Hodgkin MN, Akhtar N, MorseMA, Fuller KJ, Saqib K et al. (2001) The p85subunit of phosphoinositide 3-kinase is asso-ciated with beta-catenin in the cadherin-basedadhesion complex. Biochem J 360:335–44

Xie Z, Bikle DD (2007) The recruitment ofphosphatidylinositol 3-kinase to the E-cad-herin/catenin complex at the plasma mem-brane is required for calcium-inducedphospholipase C-gamma 1 activation andhuman keratinocyte differentiation. J BiolChem 282:8695–703

Yamamoto Y, Aoyama Y, Shu E, Tsunoda K,Amagai M, Kitajima Y (2007) Anti-desmo-glein 3 (Dsg3) monoclonal antibodies de-plete desmosomes of Dsg3 and differ in theirDsg3-depleting activities related to patho-genicity. J Biol Chem 282:17866–76

Young P, Boussadia O, Halfter H, Grose R, BergerP, Leone DP et al. (2003) E-cadherin controlsadherens junctions in the epidermis and therenewal of hair follicles. EMBO J 22:5723–33

Yuspa SH, Kilkenny AE, Steinert PM, Roop DR(1989) Expression of murine epidermal dif-ferentiation markers is tightly regulated byrestricted extracellular calcium concentra-tions in vitro. J Cell Biol 109:1207–17

Zenz R, Wagner EF (2006) Jun signalling in theepidermis: from developmental defects topsoriasis and skin tumors. Int J Biochem CellBiol 38:1043–9

Zhang JY, Green CL, Tao S, Khavari PA(2004) NF-kappaB RelA opposes epidermalproliferation driven by TNFR1 and JNK.Genes Dev 18:17–22



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